Cycles - Bake - Black patches
The object (s4studio_1) in front of your "photography" generates black "patch", it causes graininess (noise) as well.

The object is disabled from Global Viewport, but not from Rendering (Bake).
Disabling the object from Render solves the issue.





The darker appearance of photography can be solved by changing value of Mix Shader from 0.7 to 0.5, but for more info of mixing Diffuse/Glosy try this
Confusing problem when I&#39;m trying to mix Diffuse with Glossy shader



BTW: Lightning your scene by strong "Ambient Oclusion" (1.75) doesn't seem to be a good way for some natural result. Your four lamps are not (visibly) lightning the scene anymore. Deleting them you can see any difference. In this screen only World color (HSV 0/0/0.8) and Sun strenght 3 does the job.